1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device. More particularly, the present invention relates to an in-plane switching liquid crystal display (IPS-LCD) device.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices have been identified as a next-generation display device of great value because of their low power consumption and ease in portability.
Optical anisotropy and the polarization characteristics of a liquid crystal material form the basis for driving an LCD device. Generally, an LCD device includes two substrates, which are spaced apart and facing each other, and a liquid crystal layer interposed between the two substrates. Polarizers are disposed over outer surfaces of the two substrates, respectively. Each of the two substrates includes an electrode, and the electrodes of each substrate also face each other. Voltage applied to each electrode induces an electric field between the electrodes. Alignment of the liquid crystal molecules is changed by varying the intensity or direction of the electric field. The LCD device displays a picture by varying transmittance of the light according to the arrangement (or rearrangement) of the liquid crystal molecules.
One type of liquid crystal display (LCD), active matrix LCDs (AM-LCDs), which have thin film transistors (TFTs) and pixel electrodes arranged in a matrix form, and have been the subject of significant research and development because of their high resolution and superiority in displaying moving images.
The related art liquid crystal display panel has an upper substrate, a lower substrate and a liquid crystal layer interposed therebetween. The upper substrate, commonly referred to as a color filter substrate, usually includes a common electrode and color filters. The lower substrate, commonly referred to as an array substrate, includes switching elements, such as thin film transistors and pixel electrodes.
LCD device operation is based on the principle that the alignment direction of the liquid crystal molecules is dependent upon an electric field applied between the common electrode and the pixel electrode. Thus, the alignment direction of the liquid crystal molecules is controlled by the application of an electric field to the liquid crystal layer. When the alignment direction of the liquid crystal molecules is properly adjusted, incident light is refracted along the alignment direction to display image data. The liquid crystal molecules function as an optical modulation element having variable optical characteristics that depend upon polarity of the applied voltage.
In a related art LCD device, because the pixel and common electrodes are positioned on the lower and upper substrates, respectively, the electric field induced between them is perpendicular to the lower and upper substrates. However, the related art LCD devices having the longitudinal electric field have a drawback in that they have a very narrow viewing angle. In order to solve this problem of a narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been developed. IPS-LCD devices typically include a lower substrate in which a pixel electrode and a common electrode are disposed, an upper substrate having no electrode, and a liquid crystal interposed between the upper and lower substrates. A detailed explanation of the operation modes of a related art IPS-LCD device will be provided with reference to FIG. 1.
FIG. 1 is a schematic cross-sectional view illustrating a concept of a related art IPS-LCD device. As shown in FIG. 1, upper and lower substrates 10 and 20 are spaced apart from each other, and a liquid crystal layer 30 is interposed therebetween. The upper and lower substrates 10 and 20 are often referred to as a color filter substrate and an array substrate, respectively. On the lower substrate 20 are a common electrode 22 and a pixel electrode 24. The common and pixel electrodes 22 and 24 are aligned substantially parallel to each other. On a surface of the upper substrate 10, a color filter layer (not shown) is commonly positioned between the pixel electrode 24 and the common electrode 22 of the lower substrate 20. A voltage applied across the common and pixel electrodes 22 and 24 produces an electric field 26 through liquid crystal molecules of the liquid crystal layer 30. The liquid crystal layer 30 has a positive dielectric anisotropy, and thus the liquid crystal molecules align substantially parallel to the electric field 26.
The operation of a related art IPS-LCD device follows. When no electric field is applied to the common and pixel electrodes 22 and 24, i.e., off-state, the longitudinal axes of the liquid crystal (LC) molecules are parallel and form a definite angle with the common and pixel electrodes 22 and 24. For example, the longitudinal axes of the LC molecules are arranged parallel with both the common and pixel electrodes 22 and 24.
On the contrary, when a voltage is applied to the common and pixel electrodes 22 and 24, i.e., on state, an in-plane electric field 26 that is parallel to the surface of the lower substrate 20 is produced because the common and pixel electrodes 22 and 24 are on the lower substrate 20. Accordingly, the LC molecules are re-arranged such that their longitudinal axes coincide with the electric field 26.
The result is a wide viewing angle that ranges from about 80 to 85 degrees in up-and-down and left-and-right directions from a line vertical to the IPS-LCD device, for example.
FIG. 2 is a schematic plan view illustrating an array substrate for an IPS-LCD device according to the related art. As shown in FIG. 2, a gate line 40 and a data line 42 cross each other to define a pixel region P. A thin film transistor (TFT) T is formed at the crossing of the gate and data lines 40 and 42.
A common line 44 is formed substantially parallel to the gate line 40 and is spaced apart from the gate line 40. In the pixel region P, a plurality of common electrodes 46 extend in a substantially perpendicular from the common line 44, and are substantially parallel to the data line 42. The common electrodes 46 include two first common electrodes 46a and a second common electrode 46b.The two first common electrodes 46a are adjacent data lines 42, respectively. The second common electrode 46b is disposed between the two first common electrodes 46a,and thus is located in a middle portion of the pixel region P.
A first pixel connecting line 48 is connected to the thin film transistor T. A plurality of pixel electrodes 50 extend perpendicularly from the first pixel connecting line 48, and are arrayed in an alternating pattern with the plurality of common electrodes 46. A second pixel connecting line 52 is connected to ends of the pixel electrodes 50 opposite to the first pixel connecting line 48. The second pixel connecting line 52 overlaps the common line 44. The overlapped common line 44 and the second pixel connecting line 52 form a storage capacitor CST with an insulator (not shown) interposed therebetween.
Each of spaces between the respective common electrodes 46 and the respective pixel electrodes 50 substantially corresponds to an aperture area A, where liquid crystal molecules are driven according to a lateral electric field that is parallel to a substrate. Each aperture area A may be referred to as a block. In FIG. 2, one pixel has 4 blocks, that is, 4 aperture areas A. Thus, in each pixel region P, three common electrodes 46 and two pixel electrodes 50 are arranged in an alternating pattern with each other.
FIGS. 3A and 3B are enlarged views of a region B of FIG. 2. FIGS. 3A and 3B show mainly correlation of a rubbing direction of an alignment layer and a direction of an electric field induced between common and pixel electrodes.
In FIGS. 3A and 3B, an alignment layer (not shown) is rubbed along a direction from a lower-right side to an upper-left side in the context of the figures. Liquid crystal molecules are initially arranged according to a rubbing direction of the alignment layer, which may have an angle of about 45 degrees with respect to a gate line (not shown). When voltages are applied to the common electrode and the pixel electrode, a lateral electric field is induced between the common electrode and the pixel electrode and is substantially parallel to a substrate. Therefore, the lateral electric field has a direction 56 perpendicular to the common and pixel electrodes.
In FIG. 3A, voltages of about 5V and about 8V are applied to the common electrode and the pixel electrode, respectively, and a voltage of about 8V is applied to a data line. There exists a voltage difference of about 3V between the common electrode and the pixel electrode. Liquid crystal molecules (not shown) are rearranged along the lateral electric field 56 induced due to the voltage difference to have a first direction 54. At this time, the voltages correspond to a gray image.
In FIG. 3B, voltages of about 5V and about 8V are applied to the common electrode and the pixel electrode, respectively, and a voltage of about 10V is applied to a data line. Thus, although there is a voltage difference of about 3V between the common electrode and the pixel electrode, the same as the voltage difference of FIG. 3A, an electric field between the common electrode and the pixel electrode may be substantially changed due to variation of the voltage applied to the data line. Accordingly, the liquid crystal molecules are arranged along a second direction 58, which is rotated more than the first direction 54 of FIG. 3A but is not parallel to the lateral electric field 56. Even though the same voltages are applied to the common electrode and the pixel electrode, light transmittance may be changed due to different data signals.
To solve the above problem, widths of outer common electrodes may be widened. That is, in FIG. 2, to reduce cross-talk between the data line 42 and the adjacent pixel electrode 50 and prevent light leakage, the first common electrodes 46a of FIG. 2 may have wider widths than that of the second common electrode 46b of FIG. 2. However, this causes a decreasing aperture ratio.